The invention relates to an antilock system for a road vehicle having a hydraulic multiple circuit brake system, particularly a dual circuit brake system, the wheel brake cylinder of which can be acted upon by a pressure that is generated in an output pressure space of a brake unit, such as a tandem master cylinder, and that is proportional to the actuating force. An antilock system of this type is known on the basis of Honda publication, ALB, Antilock System. which is an information bulletin of Honda Deutschland GmbH, February 1983.
In known antilock systems, a pressure modulator is assigned to each wheel brake that can be controlled. The pressure modulator is connected by a control valve to a hydraulic auxiliary pressure source, or to an unpressurized storage reservoir of the auxiliary pressure source, to control the brake pressure reduction or brake pressure build-up phases that are required in the course of an antilock control cycle.
Each of the pressure modulators consists of an elongated, cylindrical housing that is closed off by end face walls and of a modulator piston that can be slid back and forth in this housing. This modulator piston comprises a primary flange and a secondary flange that are connected with one another by a piston rod of a smaller diameter which, in a pressure-sealed way, passes through a central bore of a partition wall of the modulator housing. The partition wall, together with the piston rod, form a fixed wall defining and separating two housing chambers while another wall, of the housing chambers, is defined by the movable primary flange and the secondary flange of the modulator piston respectively. One--annulus-shaped--chamber of the pressure modulator, which is movably defined by the primary flange of the modulator piston, is the output pressure space of the pressure modulator and is connected to the wheel brake cylinder of the respective wheel brake. The other--also annulus-shaped--chamber, which is movably defined by the secondary piston, is connected with the output pressure space of the brake pressure control unit, such as a tandem master cylinder.
The primary piston and the adjacent end face wall of the modulator housing define an after-running space which is connected to the brake fluid reservoir of the brake system. The secondary piston and the adjacent end face wall of the modulator housing define a driving, or control pressure space, that can be connected alternatively to the high pressure output of the auxiliary pressure source or to its unpressurized storage reservoir, by a regulating control from magnetic valves. The modulator piston is forced into its basic position by pretensioned return springs one is between the partition wall and the primary flange and the other which acts upon the secondary flange, is adjacent the end face wall of the housing. The basic position corresponds to the maximum volume of the output pressure space of the modulator in which the primary flange engages the adjacent end face wall of the modulator housing. Within this basic position, and a small portion of the overall stroke of the modulator piston, a valve is opened to permit compensating flow between the output pressure space and the after-running space of the pressure modulator.
The output pressure of the brake pressure control device in the control annulus during a braking operation, shifts the modulator piston to reduce an output pressure space, whereby, in the connected wheel brake cylinder, a corresponding brake pressure is built up. If a locking tendency occurs at the braked wheel, the driving pressure space, that until then had been kept in an unpressurized condition, is connected to the high-pressure output of the auxiliary pressure source, thereby shifting the piston of the pressure modulator to enlarge its output pressure space and thus reduce the brake pressure in the connected wheel brake cylinder. A brake fluid volume that corresponds to the volume enlargement of the output pressure space is, at the same time, pumped back from the control annulus into the brake pressure control device. If the locking tendency at the controlled wheel decreases again, so that the brake pressure can be built up again, the driving pressure space is blocked with respect to the auxiliary pressure source and connected with its unpressurized storage reservoir. The output pressure of the brake pressure control device that exists in the control chamber then causes a shifting of the modulator piston to reduce the output pressure space and thus increase the brake pressure in the wheel brake cylinder.
The known antilock system has at least the following disadvantages:
Since, during each pressure reduction phase, the piston of the pressure modulator must be shifted against the output pressure of the brake pressure control device that exists in its control pressure space, the pressure reduction speed is always the lowest when the brake pressure that exists at the start of a control process is particularly high. This naturally is unfavorable for the effectiveness of the antilock control. This disadvantage could be at least partially compensated by the fact that the auxiliary pressure source is designed for a particularly high output pressure level and/or that the surface of the secondary piston acting on the driving pressure space side is much larger than its effective piston surface that defines the control pressure space. This would result in unfavorable dimensions of the pressure modulator as a whole.
In addition, during each pressure reduction phase, a brake fluid volume, that corresponds to the volume enlargement of the output pressure space of the pressure modulator, is pumped back into brake pressure control device. Even if this volume is relatively low, a considerable pedal reaction will occur, pushing-back the brake pedal for a pressure reduction process and release the pedal for a pressure buildup phase. The pedal reaction intensity is at least unpleasant for the driver, even in cases where, because of his experience, he knows that this brake pedal reaction is an indication of the proper functioning of the antilock system. After an extended pressure reduction phase, in the course of which the brake pedal was pushed back practically to its initial position, another pressure buildup phase will follow that leads to the adjusting control of the maximum brake pressure. Thus occurs for example, when the control starts in a roadway area with a very low adhesion coefficient between it and the braked vehicle wheels and the vehicle then again reaches a roadway area with a very high adhesion coefficient. The drastic release of the brake pedal that occurs in this type of a situation and is connected to the brake pressure buildup, may be interpreted by the driver as a defect, such as a leak in the brake system in one of the brake circuits. The clearly noticeable pedal reaction that is connected with a response of the control and that is desirable in other respects, therefore, in a statistically significant number of braking situations, cannot be interpreted clearly and is therefore able to simulate a situation of potential danger which may confuse a driver who is familiar with the operation of a vehicle brake system.
It is therefore the objective of the invention to improve an antilock system wherein in the case of high brake Pressures, an increased brake pressure reduction speed is also ensured, and, in the case of a response of the control, confusing brake pedal reactions are avoided.
This objective is achieved as follows.
For normal brake operation, a step modulator piston is forced into its initial position and held against the effect of a strong return spring, by of the admission of output pressure of the auxiliary pressure source to the driving pressure space a minimal volume of its output pressure space that is connected with the brake circuit that can be subjected to the control. When the antilock control responds, a favorably high pressure reduction speed is achieved, since the brake pressure existing in the output pressure space of the pressure modulator, as well as the--maximal--pretensioning of the return spring, act in the same direction in the sense of an enlargement of the volume of the output pressure space of the pressure modulator for a reduction of the pressure in the wheel brake cylinder that is connected to it.
Since, moving of the piston from its initial position closes the intake valve, connecting the brake pressure control device with the primary chamber of the modulator, there is at first no effect on the brake unit and therefore also no pedal reaction in the sense of a pushing-back of the brake pedal in the direction of its initial position. A pedal reaction of this type will occur only when the piston reaches its the final position that corresponds to a maximum volume of the output pressure chamber and the brake pressure was not thereby sufficiently reduced. The pressure modulator must also be switched over to "recirculating operation", in which case the intake valve, because the pressure in the primary chamber of the pressure modulator is higher than in the output pressure space of the brake pressure control device, returns into its open position that permits the recirculating operation. Thus the brake pressure can be reduced sufficiently before the next retraction stroke of the pressure modulator piston. An effect of a brake pressure reduction control phase that is noticeable at the brake pedal will therefore occur only when a drastic reduction in the wheel brake cylinder or wheel brake cylinders, that are subjected to the control is required, and only to a "moderate" degree that corresponds to the reduction of a residual brake pressure. The pedal reaction that occurs upon a response of the antilock control to a drastic brake pressure reduction remains within a range that the driver by no means would consider as being "alarming". Therefore, when, a moderate pressure reduction is sufficient for the suppression of a locking tendency of a braked vehicle wheel, the volume expansion of the output pressure space of the modulator causes no pedal reaction. When a high pressure reduction is required by recirculating of brake fluid into the brake pressure control device, it is ensured by this operating mode that the output pressure space of the brake pressure control device cannot be "controlled to be empty". Also, if the auxiliary pressure supply breaks down, sufficient brake fluid always remains in the connected brake circuit in order to be able to continuously build up brake pressure in the connected wheel brake or wheel brakes by actuating the brake pressure control device.
By the dimensioning of the volume increase of the primary chamber of pressure modulator to be less than the maximum brake fluid volume, the response of the control at those wheel brakes at which the brake pressure is controlled by the pressure modulator, does not lead to a pedal reaction in a majority of braking situations. The occurrence of a significant pedal reaction is limited to that small number of cases in which, starting from a high initial brake pressure, this brake pressure must be lowered drastically, i.e., those braking situations which must be advantageously indicated to the driver as being situations of potential danger.
A bypass valve that opens when the piston of the pressure modulator reaches its position of maximum volume of its output pressure space, ensures that, in the case of a breakdown of the auxiliary pressure source, brake pressure--with a prolonged pedal path--can continuously be built up by actuating the brake pressure control device.
Simple and operationally reliable structure of the bypass valve of the pressure modulator that can be implemented as an alternative or in combination include a spring load closing element related to the step piston.
With a pressure modulator having dual driving pressure spaces and appropriate valves as an alternative to the afore-mentioned development, no lengthening of the pedal path occurs in the case of a breakdown of the auxiliary pressure source. Also a bypass valve is not required in order to ensure the operability of the brake system when the auxiliary pressure source breaks down.
The output signal of an electromechanical position indicator may be utilized in many ways, for example, for the recognition of a breakdown of the auxiliary pressure source and/or for the determination of the pressure reduction speed in a corresponding control phase of the antilock system. A signal, that represents a measurement of this pressure reduction speed, can be obtained by the time-related differentiation of the output signal of the position indicator. Using the position output signal of the position indicator, "recirculating pumping motions" of the pressure modulator piston may also be controlled in an appropriate way. This also applies to the control of pressure buildup phases of the antilock control.
In order to be able to obtain a position signal that has a sufficiently low error, the modulator piston includes a conical shell surface that tapers in the axial direction with a pin that rides on the surface and moves radially. This provides the advantage that the annulus, that is conical on the interior and remains between the piston, flanges of the modulator piston can be utilized as an unpressurized separating space. If necessary, this space provides an effective hydraulic separation of the pressure media of the brake circuit and that of the hydraulic driving circuit that may differ with respect to their physical-chemical structure. The driving circuit of the pressure modulator may then be operated by a conventional hydraulic oil. Thus it becomes possible to utilize as the auxiliary pressure source, a pressure supply unit that is provided at the vehicle for other purposes and that is equipped, for example, with a pump that is permanently driven by the engine of the vehicle and that--because of the permanent drive--must be operated with a hydraulic medium of high lubricity.
By using a check valve for the intake valve of the pressure modulator, the recirculating pumping function for the connected brake circuit is achieved in a simple way. In the recirculating operation of the pressure modulator, brake pressure control valves, assigned individually to the wheels brakes, are controlled into their blocking positions.
By using a check valve between the control valve and the auxiliary pressure source, the modulator piston, also in the case of a temporary switching-off or a breakdown of the auxiliary pressure source for a minimum duration, remains in its position that is connected with the minimum volume of the primary chamber of the pressure modulator. In this case, if an electrically operated pressure supply pump is provided, a protective operation of that pump, in intervals, is possible in response to output signals of the position indicator.
In the case of a breakdown of the auxiliary pressure source that can be recognized by a position output signal of the position indicator, characteristic of a receding of the pressure modulator piston, the control valve is moved to its position allowing volume enlargement of the primary chamber to ensure that braking can take place with a sufficient braking deceleration while the pedal path is prolonged. In which case, the pressure maintaining phases of the antilock control continue to be possible, but with no pressure reduction phases of that antilock control.
With the use of a second restoring spring, a measuring of the pressure in the driving pressure space of the pressure modulator by the position indicator is possible. Thus in an uncomplicated way, a feeding of pressure into this driving pressure space can be controlled according to the requirements.
By forming the piston in two parts, a position indicator that monitors the piston position can be utilized for measuring the pressure in the primary chamber of the pressure modulator, i.e., for measuring the brake pressure directed into the connected wheel brakes. This brake pressure information can also be used for a control of the antilock control cycles according to the requirements.
By designing the auxiliary pressure source with an overflow valve, also controlled by the position of the piston, in such a constructively uncomplicated way that it may be integrated into the pressure modulator, an overall high operational reliability of the antilock system according to the invention is achieved.
By apportioning pressure in the first pressure buildup phase that follows a pressure reduction phase of the antilock control as a function of the previous pressure reduction, a fast adaptation of the brake pressure to the respective most favorable value is achieved. As a result, a considerable gain is achieved in brake deceleration as well as in directional control.
The control signals, that in this case are required for the control of the antilock system control valve arrangement, may be obtained in a simple way from processing the position signals to determine pressure reduction.
By using a throttle in the control circuit of the pressure modulator, it is possible to place the variation rates of the brake pressures in the pressure reduction and pressure buildup phases of the antilock control, in an optimal relationship with respect to one another.
Other objects, features and advantages of the present invention will become more apparent from the following description when taking with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.